Ultrasonic imaging catheters

ABSTRACT

An intravascular ultrasonic imaging catheter is provided with a flexible circuit electrically coupled to a transducer array mounted on the distal end of the catheter, a portion of the flexible circuit being helically wound about the catheter in order to enhance the flexibility of the circuit. The catheter may be a balloon catheter which is also provided with a stent mounted on the balloon, the stent carrying one or more drugs designed to be eluted or washed into a patient&#39;s blood stream after the stent has been delivered, by a the balloon catheter, into a target area within the patient&#39;s vascular system.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional application of co-pending U.S. patentapplication Ser. No. 11/413,730, filed Apr. 28, 2006, which is aContinuation of U.S. patent application Ser. No. 10/333,099, filed Aug.6, 2003, now U.S. Pat. No. 7,037,269, which is a National Stage Entry ofinternational patent application number PCT/GB01/03262, filed Jul. 20,2001, which claims priority to Great Britain application number0017674.3, filed Jul. 20, 2000, the disclosure of which is incorporatedherein by reference.

This invention relates to ultrasonic imaging catheters includingcatheters which are combined with a balloon device.

Catheters having an ultrasonic transducer array mounted on the distalend of the catheter are known. Examples are the arrangement disclosed inthe present applicant's United Kingdom patent numbers: 2 208 138; 2 212267; 2 233 094 and 2 246 632.

Catheters fitted with a balloon which can be distended in order to pressplaque in a patient's artery back into the wall of the artery (aprocedure known as angioplasty) in order to increase the cross sectionof the artery's lumen are also well known.

In the applicant's published United Kingdom patent No. 2 325 020 thereis disclosed a catheter which has the combination of an ultrasonic arrayat or near it's distal end combined with a balloon arrangement alsocarried at or near the distal end of the catheter. With the arrangementdisclosed in the applicant's United Kingdom Patent No. 2 287 375 theultrasonic imaging system includes a multiplexing arrangement which isalso mounted on the catheter at or near it's distal end. Thisspecification also discloses a method of manufacturing thetransducer/multiplexer arrangement which method involves firstmanufacturing the arrangement in a flat configuration and then wrappingit into a cylindrical circular cross-section configuration. Thetransducer array and multiplexer arrangement are linearly separated fromone another but electrically inter-connected by what will be referred toas a flex circuit. The transducer array is cylindrical in its finalconfiguration as is the multiplexer arrangement, the latter consistingtypically of four multiplexer units, arranged in a cylindrical boxsection.

The catheters to which the present invention relates are typicallyaround one millimeter in external diameter. In the case where thecatheter also incorporates an angioplast balloon and this balloon, wheninflated, might typically have an external diameter of threemillimeters.

There is a requirement to be able to provide the ultrasonic imagingarrangement previously described either alone or in combination with aballoon arrangement previously described in a catheter having a smalleroutside diameter. More specifically, in this art the diameter ofcatheters is expressed in terms of units known as “french” and thepresent invention is concerned with providing a combined ultrasonicimaging system/balloon catheter arrangement suitable for use with a2.4-2.9 french combination catheter. (1 French equals 0.013 inches)

There is a requirement that a stent can be pre-mounted onto the balloonand firmly attached so it does not detach.

There is a requirement that the combination catheter should smoothlynegotiate a six french guide catheter and a blood vessel with a sevenmillimeter bend radius and it should do so without damage to theelectrical circuitry and components associated with the transducer.

The following requirements/constraints apply when designing a catheterhaving an ultrasonic transducer array, an associated multiplexerarrangement, a balloon and a stent inserted on the balloon:

-   a) the balloon needs to be close to the ultrasonic array at the    catheter's distal end from a visualisation point of view;-   b) the multiplexer needs to be close to the ultrasonic array in    order to minimise the length of the electrical connections between    them and to enhance the rigidity of the distal tip of the catheter;-   c) locating the mulitplexer within the balloon to meet the    requirement of b) means that it needs to be electrically insulated    from the saline solution used to inflate the balloon;-   d) the stent needs to be securely mounted on the outside of the    balloon so that it cannot be dislodged and can be radially expanded    by inflation of the balloon;-   e) one way to secure the stent on the outside of the balloon is to    crimp it in position;-   f) crimping the stent around the balloon can cause damage to the    balloon and anything contained within it.

The present invention is concerned with designing the catheter in such away that these conflicting design requirements are substantially met.

According to a first aspect of the present invention a catheter balloonspecified in the first aspect above is characterised in that themultiplexer units are longitudinally separated from the transducer, anda balloon device is positioned between the most distal multiplexer andthe transducer.

According to a second aspect of the present invention a catheter havingan ultrasonic transducer including a plurality of multiplexer units ischaracterised in that the multiplexer units are longitudinally separatedfrom one another along the length of the catheter. The catheter may alsoinclude a balloon device.

According to a third aspect of the present invention a catheter has anultrasonic transducer arrangement located at or near its distal end,which arrangement includes a flexible circuit through which electricalpower can be supplied to the transducer array, characterised in that theflexible circuit incorporates at least one helically wound section soconstructed as to enhance the flexibility of that circuit and itsability to accommodate bending as the catheter is moved along anon-linear path such as an artery of a patient. The catheter may alsoinclude a balloon device.

According to a fourth aspect of the present invention the catheterarrangement specified in the first aspect above is characterised in thatthere are flex circuits between adjacent longitudinally separatedmultiplexer units, those flex circuits being characterised in that theyare helically wound in order to enhance their flexibility and theoverall catheter assembly's capability of negotiating bends in anon-linear path without damage being caused as a result of the bending.

According to a fifth aspect of the present invention a catheter whichincludes an inflatable balloon device, an ultrasonic transducer arrayand a flexible electrical circuit adapted to supply current to thetransducer array is characterised in that at least a portion of theflexible circuit is located within the balloon device and, in use, isdirectly exposed to the inflating fluid introduced into the balloondevice, i.e. that portion of circuit is not enclosed in a protectivetube but is insulated to function when immersed.

According to a sixth aspect of the present invention a stent is crimpedover the balloon, such that the crimping force does not damage themultiplexer and transducer, and the helical flex circuit limits theminimum diameter the stent is crimped to.

How the invention may be carried out will now be described by way ofexample only and with reference to the accompanying drawings in which:

FIG. 1 is a schematic representation of a combination catheterincorporating the present invention;

FIG. 2 is a fragmentary view on a larger scale showing a balloon device,ultrasonic transducer array and associated multiplexer arrangementforming part of the overall arrangement shown in FIG. 1;

FIG. 3 is a side elevational fragmentary view on an enlarged scaleillustrating a guide wire exit to a catheter having an inner and outerbody at its distal end;

FIG. 4 comprises five cross-sectional views, FIGS. 4A, 4B, 4C, 4D and 4Etaken on the lines A-A, B-B, C-C, D-D and E-E respectively of FIGS. 1and 3.

FIG. 5 is a sectional elevational view on an enlarged scale showing thatportion of the catheter which comprises the transducer array adjacentthe distal end of the balloon device, both balloon and transducer beinglocated at or near the distal end of the catheter itself;

FIG. 6 comprises FIG. 6A which shows that part of the catheter whichincludes the transducer array, flex circuit and multiplexers when theyare in the flat condition and FIG. 6B which shows the same arrangementwhen they are in the wrapped condition; and

FIG. 7 is an enlarged prospective view of a drug loaded stent accordingto the present invention.

FIG. 1

This is a diagrammatic illustration of the overall catheter arrangementto which the present invention is applied.

The detailed construction is shown in the other figures of the drawingsin which the same reference numerals have been used to designate thesame or equivalent elements.

In FIG. 1 a catheter generally indicated at 1 has at its distal end Athe combination of a balloon unit 2 and an ultrasonic annular transducerarray 3, the extreme distal end of the catheter comprising a soft tip 4.

Associated with the transducer array 3 is a multiplexing arrangementindicated at 5.

At the proximal end B of the catheter there is provided a ballooninflation port 6 by which fluid (typically a saline solution 19) can beintroduced through the catheter into the balloon 2 in order to inflateit in known manner.

Also at the proximal end B there is provided a connector 7 andassociated strain relief device 8 by which the proximal end B of thecatheter may be connected to a catheter interface module (CIM) and thusto the electronic imaging system.

At an intermediate point along its length the catheter is provided withopaque markers 9 to assist the clinician in being able to see thecatheter within the patient's artery using x-ray equipment.

The ultrasonic transducer array 3 is provided with electrical power bymeans of a ribbon cable 22 which runs the length of the catheter, theproximal end of the ribbon cable being connected to an electrical supplyand control arrangement (not shown) which itself is not part of thepresent invention.

The multiplexing arrangement 5 consists of a number of multiplexingunits, in this case four, the functional purpose of which is to reducethe number of electrical leads which would otherwise have to be providedalong the length of the catheter in order to energise the large numberof transducer elements in the transducer array 3. Typically the numberof elements would be sixty four. By having a multiplexing arrangementthe number of electrical leads can be significantly reduced. Theprovision of such a multiplexing arrangement is known.

The present invention is concerned with the configuration andconstruction of the multiplexing arrangement and the associated flexiblecircuit and the relationship of the balloon device to the flexiblecircuit.

According to the present invention the individual multiplexing units 5a, 5 b, 5 c and 5 d are longitudinally spaced from one another along thelength of the catheter as indicated in FIG. 1, the longitudinal spacingbeing identified as “S”.

In the arrangement of FIG. 1 the longitudinally spaced multiplexingunits 5 are located immediately proximal to the balloon device 2 whichin turn is proximal to the ultrasonic transducer array 3.

Mounted on the balloon is an optional stent 102 which is crimped ontothe balloon. The helical flex circuit 12 acts as a limit to the crimpingof the stent. The crimping operation involves application of a forcewhich would otherwise damage components inside, but in this design thecomponents are outside the stent region.

The construction of this distal end of the catheter is illustrated inmore detail in FIGS. 2 to 5.

FIGS. 2 TO 5

The ultrasonic transducer array 3 consists of sixty-four individualtransducer elements arranged in a cylindrical configuration, theseelements being contained between a proximal ring 24 and a distal ring 25(FIG. 2). The multiplexer units 5 a, 5 b, 5 c and 5 d are electricallyconnected to the transducer array 3 by means of a flexible circuitindicated at 12.

This flexible circuit 12 is arranged in a helical configuration and itpasses from the transducer array 3 to the multiplexer units 5 throughthe balloon device 2.

The balloon device 2 comprises a flexible and expandable balloonenvelope 13 which is sealed at 14 and 15 to the catheter.

That portion of the flexible circuit 12 which passes through the insideof the balloon unit 2 is insulated with a water proof coating such asParylene™ (Specialty Coatings Ltd, Northampton) and exposed to the fluid(typically a saline solution 19) which is used to inflate the balloonenvelope 13, i.e. that part of the flexible circuit 12 which is withinthe balloon envelope 13 is not contained within any protective tube.

At its distal end the catheter 1 consist of an inner body or tube 16 andan outer body or tube 17.

The catheter is inserted into a patient's artery after a metal guidewire 18 (see FIG. 3) has first been inserted into the artery. Thecatheter then in effect runs down the guide wire to bring the distal endof the catheter into the target area within the patient.

More specifically the catheter is loaded onto the proximal end of theguide wire 18 by the clinician who pushes the inner body 16 over theproximal end of the guide wire and then feeds the catheter down theguide wire. As this feeding operation occurs the guide wire 18 in effectpasses outside the catheter at the guide wire exit indicated at 20, inFIG. 3, the guide wire exit 20 being formed by the end of the inner body16.

The proximal end of the outer body 17 is sealingly secured to an outertube 1 a of the catheter which contains an inner tube 1 b, typically astainless steel hypodermic, the tubes 1 a and 1 b running the length ofthe catheter 1 up to a Y-connector 101.

A tapered metal member or wire known as a stilett 21 extends from thedistal end of the inner tube 1 b into the space between the inner andouter bodies 16 and as illustrated in FIG. 3. The purpose of the stilett21 is to provide a support for the guide wire exit port which wouldotherwise have a tendency to kink.

FIG. 4

This figure comprises FIGS. 4A, 4B, 4C, 4D and 4E which arecross-sections taken on the lines G3, G2, G1, respectively in FIGS. 3,and F and E respectively in FIG. 1.

As can be seen from these cross-sections the ribbon cable 22 consists ofa number of electrical leads which for most of its length are mouldedtogether to form the unit shown in FIGS. 4A, 48 and 4C. However, at thepoint where it is required to connect electrically the variousconstituents of the ribbon cable to the four multiplexer units 5 theribbon cable 22 is split into discreet leads as shown in FIG. 4D. Theseleads are then connected to the respective multiplexer units via trackson the flex circuit.

The manner in which each multiplexer unit is mounted is shown in FIG.4E. Each multiplexer unit is secured to the inner body 16 by means ofadhesive 23.

FIG. 5

This shows on a greater scale and in more detail the extreme distal endof the catheter, the same reference numerals being used to designateparts already described with reference to earlier figures.

The annular ultrasonic transducer array 3 is contained between aproximal ceramic ring 24 and a stainless steel or ceramic distal ring25. At the distal end of the distal forming ring 25 there is the softtip 4, which is typically made from Nylon® which is heat melted or fusedonto the distal end of the inner body 16.

FIG. 6

FIG. 6A shows that part of the catheter which includes the transducerarray 3 and multiplexers 5 when they are in the flat condition at anintermediate point in the manufacturing process. A more detaileddisclosure of the method of manufacturing this arrangement, whichinvolves first constructing the transducer and multiplexer assembly inthe flat and then converting it into a tubular configuration isdisclosed in more detail in our United Kingdom Patent No. 2 297 375

FIG. 6B shows the same arrangement as FIG. 6A but after it has beenformed into the cylindrical configuration referred to earlier.

FIG. 7

The present invention also envisages a stent, such as that shown at 102in FIG. 1, being loaded with one or more of a variety of drugs so thatthe drug or drugs is/are eluted or washed from the stent by thepatient's blood flowing past the stent.

By having a drug loaded stent mounted on an intravascular ultrasonicimaging catheter (IVUS) it is possible for the clinician to moreaccurately position the stent and target the drug where it is requiredin order to prevent, for example, restenosis. The usual method ofintroducing a drug or drugs has been to simply introduce them generallyinto the patient's blood stream. However, this means that a largeproportion of the introduced drug is in effect wasted and is notoperative in the target area.

The loading of the drug or drugs onto the stent can be achieved in anumber of ways.

A drug loaded surface of a stent can be achieved by using differenttechnological approaches. Each of these approaches can be conducted in away that the drug compound is released from the surface either in ashort (hours) or an extended time frame (days). The release kinetics canbe adjusted by applying specific modifications to the surface of thestent e.g. hydrophobic or hydrophilic side chains of a polymer carrieror a ceramic surface.

The following outlines four possible ways of loading the drug/drugs ontothe stent.

1. Ceramic Coating

An AlO₂ coating (patent applications DE 19855421, DE 19910188, WO00/25841) with porous surface can be loaded with a drug in amountsbetween 250 μg an 10 mg by either dipping, spraying or similartechniques. The drug dose is dependent on the nature of the targetvessel and the condition of the patient and is chosen such that cellproliferation is sufficiently inhibited, while healing is not hampered.The drug can be used as an aqueous or organic solution, e.g. in DMSO,DMF and methanol. After spraying or dipping (under mild vacuum) thetreated stent is dried, the procedure is repeated three to ten times.After the final drying the stent is rinsed for one minute in water orisotonic saline at room temperature and then dried again. Drug contentcan be analysed by standard methods (HPLC, LC-MS) after eluting orwashing the compound with a suitable solvent. Release kinetics can beanalysed using a standard release apparatus.

2. ePTFE Membrane: Stent Graft

Identical approach as above. The drug is absorbed into the cavities ofthe porous ePTFE membrane.

3. Polymeric Coating in General

Different polymers are suitable for drug loading: methacrylate-polymers,polyurethane-coatings, ePTFE-coatings. The drug can be either applied tothe final surface (see above) or directly added to the polymerisationsolution.

4. Mechanical Approach (FIG. 7)

The mechanical approach is based on holes 701 that have been introducedinto the stent 700 struts 702 using a laser. These holes can then befilled with a drug or drugs. The hole-approach can be combined with athin, biodegradable coating that in itself is drug loaded. After initialrelease from the biodegradable coating the drug-filled holes can servefor long term release of active drug. Interstices for containing thedrug may be formed in other ways than by holes.

A variety of drugs that could be loaded onto the stent are listed in thefollowing three tables, Table A, Table B and Table C. It is intendedthat the listed drugs should also include their derivatives.

In this example the drugs are selected to be active in three phases,Table A being Phase I, Table B being Phase II and Table C being PhaseIII.

Phase I is aimed at effecting vasodilation e.g. the dilation of thepatient's artery. The effects are listed in the table.

Phase II is aimed at inhibiting inflammation, etc as listed at the topof Table B. Again the effects of the drugs are set out in the table.

Phase III is aimed at the inhibition of cell proliferation and again theeffects of the drugs are set out in the table.

TABLE A Phase I - Vasodilation Drugs to be released with the first 24-72h after stenting Drug name Rationale/Effects Priority Molsidomine,linsidomine, Release of NO leads to vasodilation, 1 sodiumnitroprusside, reducing the degree of procedural vessel nitroglycerol,NO-donors in wall damage, stimulates the growth of general endothelialcells, inhibits migration and proliferation of smooth muscle cells.Stimulators of the soluble SGC stimulators induce vasodilation and 1guanylate cyclase like BAY 41- other NO-effects by directly activatingthe 2272 (5-(Cyclopropyl-2-[1-(2- target enzyme of NO.fluoro-benzyl)-1H-pyrazolo[3,4- n]pyridin-3-yl]-pyrimidin-4- ylamine).Hydralazine. Causes smooth muscle cell relaxation. 2 Verapamil,dilitazem, nifedipine, Smooth muscle cell contraction reduced 3nimodipine and other Ca²⁺ - by reducing intracellular Ca²⁺ - channelblockers. concentrations. Captopril, enalapril, lisinopril, Reduction ofthe angiotensin II level leads 1 quinapril and other angiotensin toreduced vasoconstriction. converting enzyme inhibitors. Losartan,candesartan, Reduced vasoconstriction is achieved by 1 irbesartan,valsartan and other blocking the effect of angiotensin II at itsangiotensin II receptor target receptor located in the vascularantagonists. tissue.

TABLE B Phase II - Inhibition of inflammation, immunosuppression,promotion of endothelial cell growth, inhibition of cell migration.Drugs to be released within the first 2-21 days after stenting. Drugname Rationale/Effects Priority Dexamethasone, Inhibition ofinflammatory reactions by 1 Betamethasone, prednisone different effectson macrophages and and other corticosteriods monocytes, endothelialcells, basophils, fibroblasts and lymphocytes. 17-beta-estradiolInhibition of migration and proliferation of smooth muscle cells. FK506(Tacrolimus) Inhibition of T-cell response, reduction of pro- 1inflammatory cytokine release, inhibition of smooth muscle cellproliferation Cyclosporine Inhibition of T-cell response 3 Mycophenolicacid Inhibition of B-cell response, inhibition of 3 smooth muscle cellproliferation VEGF, VEGF-receptor VEGF is a growth factor stimulatingthe 1 activators growth of smooth muscle cells Tranilast Has shownefficacy (prevention of restenosis) 2 in animal trials after systemicapplications, inhibits keliod scarring. Mefoxicam, cefebrex, vioxxAntiinflammatory effect through inhibition of 2 and other COS-2antagonists cyclooxygenase 2 Indomathacin, diclofenac, Antiinflammatoryeffect through inhibition of 3 ibuprofen, naproxen and cyclooxygenase1,in addition platelet other COX-1 inhibitors inhibition Plasminogenactivator Inhibits activation of prourokinase. 1 inhibitor-1 and otherserpins Prourokinase promotes cellular migration by acctivating plasminand metalloproteinases as well as other proteinases Thrombin inhibitorsas Thrombin promotes thrombus formation but is 2 hirudin, hirulog,agratroban, also a strong mitogen (growth factor) PPACK etc.Interleukin-10 Antiinflammatory cytokine that inhibits 3 monocytes

TABLE C Phase III - Inhibition of cell proliferation Drugs to bereleased within the first 14 days to 3 months after stenting Drug nameRationale/Effects Priority Sirolimus, SDZ RAD (40-O- Inhibition ofT-cell response, reduction of 1 (2-hydroxyethyl)-rapamycinpro-inflammatory cytokine release, inhibition and other rapamycindrivatives of smooth muscle cell proliferation PDGF-antoginistsInhibition of smooth muscle cell proliferation 1 and migration throughinhibition of PDGF- signal transduction. PDGF is a strong mitogen forsmooth muscle cells. Paclitaxel Inhibition of smooth muscle cellproliferation 1 through promotion of microtubili association CisplatinInhibition of smooth muscle cell proliferation 2 through intercalationin DNA-double strand Vinblastin Inhibition of smooth muscle cell 2proliferation through inhibition of mitotic spindle formationMitoxantrone Inhibition of smooth muscle cell proliferation throughinhibition of DNA and RNA synthesis and inhibition of topisomerase II 1Combretastatin A4 Inhibition of smooth muscle cell 1 proliferationthrough inhibition of mitotic spindle formation Topotecan Inhibition ofsmooth muscle cell 2 proliferation through inhibition of topoisomerase IMethotrexate Inhibition of smooth muscle cell 3 proliferation throughinhibition of dihydrofolate reductase Flavopiridol Inhibition of smoothmuscle cell 1 proliferation through inhibition of cell cycle kinase

The invention claimed is:
 1. A method of manufacturing an apparatus forultrasonic imaging, the method comprising: providing a catheter having adistal region; coupling an annular ultrasonic imaging transducer arrayto the distal region of the catheter such that the annular ultrasonicimaging transducer array extends annularly about a longitudinal axis ofthe catheter, wherein the ultrasonic imaging transducer array is fixedlysecured to the distal region of the catheter in the annularconfiguration such that the ultrasonic imaging transducer array extendsannularly about the longitudinal axis of the catheter when the distalregion of the catheter extends linearly along the longitudinal axis ofthe catheter; electrically coupling a flexible circuit to the transducerarray and helically winding at least a portion of the flexible circuitabout the catheter, the flexible circuit configured to supply electricalpower to the transducer array; and electrically coupling a plurality ofmultiplexers to the flexible circuit proximal of the transducer arraysuch that a section of the helically wound portion of the flexiblecircuit is disposed between the transducer array and the plurality ofmultiplexers and such that the plurality of multiplexers arelongitudinally separated from one another along the longitudinal axis ofthe catheter.
 2. The method of claim 1, wherein the flexible circuit isinsulated.
 3. The method of claim 1, wherein the resulting apparatus hasa maximum external diameter of less than about 3 Fr.
 4. The method ofclaim 1, wherein the resulting apparatus is adapted for passing within aguiding catheter having a lumen diameter of 6 Fr or larger.
 5. Themethod of claim 1, further comprising communicatively coupling animaging system to the resulting apparatus.
 6. A method of manufacturingan apparatus for ultrasonic imaging, the method comprising: providing acatheter having a distal region; coupling an annular ultrasonic imagingtransducer array to the distal region of the catheter, wherein theultrasonic imaging transducer array is fixedly secured to the distalregion of the catheter in the annular configuration such that theultrasonic imaging transducer array extends annularly about thelongitudinal axis of the catheter when the distal region of the catheterextends linearly along the longitudinal axis of the catheter;electrically coupling a flexible circuit to the transducer array andhelically winding at least a portion of the flexible circuit about thecatheter, the flexible circuit configured to supply electrical power tothe transducer array; electrically coupling a plurality of multiplexersto the flexible circuit proximal of the transducer array such that thehelically wound portion of the flexible circuit is disposed between thetransducer array and the plurality of multiplexers; and coupling aballoon to the catheter, wherein the balloon is coupled to the catheterproximal of the transducer array.
 7. The method of claim 6, wherein theplurality of multiplexers are positioned proximal of the balloon.
 8. Amethod of manufacturing an apparatus for ultrasonic imaging, the methodcomprising: providing a catheter having a distal region; coupling anultrasonic imaging transducer array to the distal region of the cathetersuch that the ultrasonic transducer array extends annularly around alongitudinal axis of the catheter, wherein the ultrasonic imagingtransducer array is fixedly secured to the distal region of the catheterin the annular configuration such that the ultrasonic imaging transducerarray extends annularly about the longitudinal axis of the catheter whenthe distal region of the catheter extends linearly along thelongitudinal axis of the catheter; electrically coupling a flexiblecircuit to the transducer array and helically winding at least a portionof the flexible circuit about the catheter, the flexible circuitconfigured to supply electrical power to the transducer array;electrically coupling at least one multiplexer to the flexible circuit;and coupling a balloon to the catheter, wherein the balloon is coupledto the catheter proximal of the transducer array; wherein the at leastone multiplexer is positioned proximal of the balloon; and wherein theat least a portion of the flexible circuit is helically wound about thecatheter between the transducer array and the at least one multiplexer.9. The method of claim 8, wherein the at least a portion of the flexiblecircuit is disposed within the balloon.
 10. The method of claim 9,wherein the at least a portion of the flexible circuit disposed withinthe balloon is insulated.
 11. The method of claim 8, further comprisingposition a stent coaxially about the balloon.
 12. The method of claim 7,further comprising transitioning the transducer array from a flatconfiguration to a cylindrical configuration.
 13. The method of claim 8,wherein electrically coupling at least one multiplexer to the flexiblecircuit includes electrically coupling a plurality of multiplexers tothe flexible circuit.
 14. The method of claim 13, wherein the pluralityof multiplexers are positioned proximal of the transducer array suchthat the helically wound portion of the flexible circuit is disposedbetween the transducer array and the plurality of multiplexers.
 15. Themethod of claim 14, wherein the balloon is positioned proximal of thetransducer array and distal of the plurality of multiplexers such thatat least a portion of the helically wound portion of the flexiblecircuit extends through the balloon.
 16. The method of claim 14, whereinthe plurality of multiplexers are separated from one another along thelongitudinal axis of the catheter.